The present invention relates to a solar thermal system and, more particularly, to a solar thermal system wherein solar heat is utilized in a state of saturated steam or superheated steam with a pressure close to 1 atmospheric pressure.
FIG. 5 is a perspective view partly in section schematically showing the principal part of a conventional solar thermal system, and in the figure, reference numeral 51 represents an absorbing plate. The absorbing plate 51 is made of a stainless steel plate, an aluminum plate, or the like, almost in the shape of a rectangle in a plan view. A black coating (not shown) is applied to the top 51a of the absorbing plate 51, whereby solar heat can be easily absorbed. Multiple flow tubes 52, made of a metal such as copper or stainless steel almost in the shape of a tube, are arranged in parallel at prescribed places on the absorbing plate 51, and the tube walls of the flow tubes 52 and the absorbing plate 51 are united in a body by welding or the like. Headers 53 and 54 almost in the shape of a hollow quadrangular prism are integrally joined to both ends of the flow tubes 52 at the side walls 53a and 54a, and hole portions of the flow tubes 52 communicate with hollow portions of the headers 53 and 54 (neither shown). A case 55 almost in the shape of a box shown by a chain line is arranged around the absorbing plate 51, the flow tubes 52, and the headers 53 and 54, and the space between the inner walls of the case 55, and the absorbing plate 51 and the headers 53 and 54 is filled with an insulation (not shown) such as glass wool. A transmission body 56 is arranged above the absorbing plate 51 and the flow tubes 52 so as to cover them, and the transmission body 56 is made of reinforced glass, a transparent plastic material, or the like which easily transmits solar light almost in the shape of a board. The transmission body 56 is closely fitted to the upper portion of the case 55 through packing (not shown), and the distance D between the transmission body 56 and the absorbing plate 51 or the flow tubes 52 is set as short as possible in order to prevent a heat loss caused by convection. A flat-plate solar thermal collector 50 comprises these absorbing plate 51, flow tubes 52, headers 53 and 54, case 55, transmission body 56, and associated parts.
One end portion 54b of the header 54 is connected through a supply pipe 57a, a pump 57c, a selector valve 57d, and a feed water pipe 57e, to a feed water tank (not shown). In addition, to the selector valve 57d, one end of a supply pipe 57b is connected, while the other end thereof is connected to the lower portion inside a vessel 61. A medium supply system 57 comprises these supply pipes 57a and 57b, pump 57c, selector valve 57d, feed water pipe 57e, and associated parts.
The vessel 61 made of a metal almost in the shape of a hollow rectangular parallelepiped is arranged at a prescribed place below the flat-plate solar thermal collector 50, an insulating member 62 is fitted around the vessel 61, and the outer surface of the insulating member 62 is protected by a protective member 63 made of a metal plate. An air vent portion 61a is formed at a prescribed place in the upper portion of the vessel 61, so that air in the vessel 61 can easily flow out or in through the air vent portion 61a with changes in volume of high temperature water 65 within the vessel 61. One end of a discharge pipe 64a is connected to a prescribed place in the upper portion of the vessel 61, while the other end thereof is connected through a valve 64b to one end portion 53b of the header 53. A heat storage means 60 comprises these vessel 61, insulating member 62, protective member 63, piping 64a, valve 64b, and associated parts.
One end of a piping 66 is fitted to a prescribed place inside the vessel 61, while the other end thereof is connected through a pump to heat-using equipment (neither shown) such as a heat exchanger, a heat pump, a feed water heater, a bathtub, a heating apparatus, and an absorption refrigerating machine. A solar thermal system comprises these flat-plate solar thermal collector 50, medium supply system 57, heat storage means 60, heat-using equipment, and associated parts.
In the use of the solar thermal system with the above construction, the flat-plate solar thermal collector 50 is set and fixed in a prescribed direction and at a prescribed tilt angle so that the absorbing plate 51 should be densely irradiated with solar radiation, and that the header 53 should be in a higher position than the header 54. The valve 64b is opened, the selector valve 57d is switched to the feed water pipe 57e side, and by driving the pump 57c, water is supplied at a prescribed flow rate through the piping 57e, the selector valve 57d, the pump 57c, the piping 57a, and the header 54 to the flow tubes 52 as shown by arrows in the figure. Then, solar heat absorbed by the absorbing plate 51 is transferred through the flow tubes 52 to the water by thermal conduction or the like, so that the high temperature water 65 with almost 1 atmospheric pressure and a temperature of 80-90xc2x0 C. or so is generated. The high temperature water 65 passes through the header 53, the discharge pipe 64a, and the valve 64b so as to be stored in the heat storage means 60. When the temperature of the high temperature water 65 stored in the heat storage means 60 becomes low, the selector valve 57d is switched to the supply pipe 57b side, and by allowing the water to flow through the flat-plate solar thermal collector 50 again, the temperature of the high temperature water 65 is maintained.
When a prescribed amount of the high temperature water 65 is stored in the heat storage means 60, by driving the pump of the piping 66, the high temperature water 65 is supplied to the heat-using equipment side.
FIG. 6 is a sectional view schematically showing another conventional solar thermal system, and in the figure, reference numerals 51, 52, 53-54, and 55 represent an absorbing plate, flow tubes, headers, and a case, respectively, almost the same as those shown in FIG. 5. To the tops of the absorbing plate 51, the flow tubes 52, and the headers 53 and 54, almost the same black coating (not shown) as shown in FIG. 5 is applied. The space between the inner walls of the case 55, and the absorbing plate 51 and the headers 53 and 54 is filled with an insulation 71. Transmission bodies 72a and 72b are fitted double above the absorbing plate 51 and the flow tubes 52, whereby a heat loss outward caused by conduction is restricted. Furthermore, in order to prevent a heat loss caused by convection, a clearance 72c between the transmission bodies 72a and 72b, and a gap 73 between the transmission body 72a and the absorbing plate 51 are kept at a prescribed low pressure. A flat-plate solar thermal collector 70 comprises these absorbing plate 51, flow tubes 52, headers 53 and 54, case 55, insulation 71, and associated parts.
A heat exchanger 74 is placed below the flat-plate solar thermal collector 70, and the heat exchanger 74 comprises a vessel 74a around which an insulating member (not shown) is fitted, and a coil-like inner piping 74b vertically running within the vessel 74a. The header 53 is connected through a piping 76a to the upper end of the inner piping 74b, while the header 54 is connected through a piping 76b, a compressor 75, and a piping 76c to the lower end of the inner piping 74b. On the other hand, the upper portion of the heat exchanger 74 is connected through a piping 79a to an inlet portion 77a of a steam turbine 77, being a piece of heat-using equipment. And an outlet portion 77b of the steam turbine 77 is connected through a piping 79b, a condenser 78a, a piping 79c, a circulating pump 78b, and a piping 79d, to the lower portion of the heat exchanger 74. In addition, a generator 77d is mechanically coupled to the axis of rotation 77c of the steam turbine 77.
In the use of the solar thermal system with the above construction, by driving the compressor 75 using water, for example, as a heating medium, pressurized water with a prescribed pressure and a prescribed flow rate is supplied through the piping 76c and the header 54 to the flow tubes 52 as shown by an arrow in the figure. Since the solar thermal collector 70 has a smaller heat loss caused by conduction and convection, compared with the solar thermal collector 50 shown in FIG. 5, the temperature of the pressurized water passing through the flow tubes 52 is raised to about 130xc2x0 C. The pressurized high temperature water passes through the header 53 and the piping 76a to be supplied to the inner piping 74b of the heat exchanger 74. After the temperature thereof is lowered by transfer of heat to water within the vessel 74a, the pressurized water passes through the piping 76b and the compressor 75 to be circulated again. On the other hand, the water heated in the heat exchanger 74 becomes pressure steam, which is introduced through the piping 79a to the steam turbine 77 and expands therein. The steam comes into collision with turbine blades (not shown) with developing the velocity of flow and drives the generator 77d through the axis of rotation 77c so as to generate electricity. The steam discharged from the steam turbine 77 becomes hot water in the condenser 78a, which is supplied through the circulating pump 78b to the heat exchanger 74 to be circulated again.
As described above, in the solar thermal system shown in FIG. 5, solar thermal energy is transformed to be collected as sensible heat of the high temperature water 65 (with about 1 atmospheric pressure and a temperature of 80-90xc2x0 C. or so). In this case, the amount of heat collected per unit weight of water is smaller than that in the case of latent heat collection. As a result, in order to collect large amounts of heat, it is necessary to supply and transport relatively large amounts of water to the solar thermal collector 50. Therefore, the rigidity of the apparatus and the device capacities of the pump 57c and the like must be set high, so that the apparatus is difficult to design and the cost thereof tends to be high. In addition, since the heat loss from the transmission body 56 and the like is large, the heat-collection efficiency is not good.
Though it is not shown in the figure, in a large-scale solar thermal system wherein multiple solar thermal collectors 50 are coupled and placed at the ground level and the top of the heat storage means 60 is set higher than the tops of the solar thermal collectors 50, another pump for controlling the flow rate, being interlocked with the pump 57c, needs to be placed near the valve 64b in order to store the high temperature water 65 in the heat storage means 60. As a result, the system is difficult to design, assemble, and maintain, and the cost thereof tends to be further higher.
As described above, in the solar thermal system shown in FIG. 6, solar thermal energy is transformed to be collected as sensible heat of pressurized high temperature water (with about 4-6 atmospheric pressure and a temperature of 110-130xc2x0 C. or so). The amount of heat collected per unit weight of water is smaller, similarly to the case in FIG. 5. As a result, it is necessary to supply and transport relatively large amounts of water to the solar thermal collector 70. Therefore, the rigidity of the whole apparatus and the device capacities of the compressor 75 and the like need to be set high. In addition, since the solar thermal collector 70, the heat exchanger 74, the compressor 75, and the like constitute a closed cycle and the pressure of water is high, it is necessary to make the pressure resistance much higher, compared with that in FIG. 5, and to prevent leakage of water. Moreover, since the clearance 72c and the gap 73 must be kept almost in vacuum, it tends to be difficult to design, produce and maintain the solar thermal collector 70. As a result, the cost is likely to be much higher.
The present invention was developed in order to solve the above problems, and it is an object of the present invention to provide a solar thermal system, wherein using a small amount of water as a medium, solar thermal energy can be largely and efficiently transformed to be collected as latent heat of saturated steam or superheated steam with 1 atmospheric pressure, and the generated saturated steam or superheated steam is automatically transported and can be easily and certainly stored, so that the light weight of apparatus and the downsizing of device capacities can be achieved, and that the apparatus can be easily designed, produced and maintained, resulting in a substantial reduction in cost.
The present inventors completed the present invention on the basis of the following knowledge shown in (1)-(6).
(1) When the collectable amount of heat of solar energy by a flat-plate solar thermal collector is about 500 W/m2, the theoretical amount of saturated water (with a rise in temperature from 0xc2x0 C. to 100xc2x0 C. and 1 atmospheric pressure) generated by that amount of heat is about 1.2 g/m2xc2x7s (about 12 g/m2xc2x7s in the case of a rise in temperature from 90xc2x0 C. to 100xc2x0 C.). On the other hand, the theoretical amount of saturated steam (with a temperature of 100xc2x0 C. and 1 atmospheric pressure) generated by that amount of heat is about 0.22 g/m2xc2x7s (about 370 ml/m2xc2x7s). Compared with the case of generating saturated water with a rise in temperature from 0xc2x0 C. to 100xc2x0 C., the weight of water used (circulating) can be reduced to about ⅕ (about {fraction (1/50)} in the case of a rise in temperature from 90xc2x0 C. to 100xc2x0 C.).
Therefore, when solar thermal energy is collected as latent heat of saturated steam, for example, the amount of a medium (water) supplied can be made smaller, so that it is possible to reduce the strength of apparatus and the capacities of devices, leading to cost cutting.
(2) A droplet having a radius of 10xe2x88x924 m-10xe2x88x925 m more easily absorbs radiant heat, compared with bulk water with a smooth surface. In addition, when the radius of the droplet becomes about 10xe2x88x927 m or smaller with vaporization, the saturated steam pressure becomes larger, compared with the bulk water, so that under 1 atmospheric pressure, the droplet is likely to change into steam at lower temperatures than 100xc2x0 C.
(3) When the top of saturated water and the bottom of a high temperature absorbing body face each other with steam between, the thermal radiant energy applied to the top of the saturated water is expressed by the following equation (a), where Ew is the thermal radiant energy of the saturated water, the thermal emissivity xcex5w of the top of the saturated water (regarded as a black body) is 1, xcex5c is the thermal emissivity of the bottom of the absorbing body, and Ecb is the thermal radiant energy of the black body at the same temperature as the temperature Tc of the absorbing body.
(1xe2x88x92xcex5c)Ew+xcex5cxc3x97Ecbxe2x80x83xe2x80x83(a) 
Therefore, the energy E which is transformed to latent heat by water vaporization is expressed by the following equation (b).
E={(1xe2x88x92xcex5c)Ewxcex5cxc3x97Ecb}xe2x88x92Ew=xcex5c(Ecbxe2x88x92Ew)xe2x80x83xe2x80x83(b) 
At this time, since the thermal radiant energy Ew of the saturated water is constant, E is roughly proportional to xcex5c. By making the bottom of the absorbing body a black body, it is possible to increase the amount of radiative transfer from the absorbing body to the saturated water. On the other hand, when the bottom of the absorbing body is not made a black body, the amount of radiative transfer to the saturated water greatly decreases and only the temperature of the absorbing body rises.
(4) When an absorbing body having a prescribed temperature is arranged in a horizontal position in the upper portion inside a steam generating chamber full of droplets, superheated steam with relatively high temperatures, saturated steam with about 100xc2x0 C., and saturated water with temperatures of about 100xc2x0 C. and less are distributed and dwell in this order in the downward direction from the vicinity of the bottom of the absorbing body. Since the droplets distributed in the higher positions have smaller specific gravities, natural convection is unlikely to occur. In addition, in the generation of saturated steam and superheated steam, forced convection based on changes in volume has a minimal chance of occurrence. The existence of such dwelling and distribution lowers the heat transfer efficiency from the absorbing body to the droplets.
(5) When the flow of steam in a transport system is a steady flow, the pressure difference xcex94P between the pressure within the steam generating chamber and 1 atmospheric pressure is expressed by the following equation (c), where xcex94Pu is the minimum steam pressure""difference required for making the steady flow, and xcex94P1 is a pressure loss (transport resistance) in the transport system.
xcex94P=xcex94Pu+xcex94P1xe2x80x83xe2x80x83(c) 
Therefore, when the transport system is fully thermally insulated by an insulation, by controlling the pressure difference xcex94P between the pressure within the steam generating chamber and 1 atmospheric pressure to be about a few hundred Pa, the density of steam can be almost uniform in the whole passage of the transport system. As a result, as steam is continuously generated in the steam generating chamber, almost the same amount of steam as that can be automatically pushed out and transported in sequence through the transport system. In addition, in order to make the pressure loss xcex94P1 of the transport system almost a few hundred Pa or less, the sectional form of the piping in the transport system is easily determined based on the steam transport capacity, the transport distance, and the like. As a result, the transport system can be easily designed and the resistance to pressure of the whole system can be set low.
(6) In the case of a steam storage means in the shape of a cup upside down, when saturated steam, for example, is supplied thereto, the saturated steam moves upward and is stored in the upper portion (steam storage portion) inside the steam storage means, since the specific gravity of the saturated steam is smaller than that of air, while air moves downward. Since the air is pushed up at all times with 1 atmospheric pressure through an opening, the saturated steam with 1 atmospheric pressure can be certainly stored in the steam storage portion.
In order to achieve the above object, a solar thermal system (1) according to the present invention is characterized by comprising a steam generating chamber formed between an absorbing body and a heat insulation case, wherein droplets are evaporated by solar heat, and a droplet supply means to supply droplets into the steam generating chamber, wherein steam generated in the steam generating chamber is used as a heating medium.
Here, the droplet to be supplied into the steam generating chamber by the droplet supply means is roughly spherical in shape and the radius thereof is desirably 10xe2x88x924-10xe2x88x925 m or so from the viewpoint of the power efficiency in the droplet supply means.
Using the above solar thermal system (1), since the droplets supplied by the droplet supply means fill the steam generating chamber, and the droplets more easily absorb radiant heat energy emitted by the absorbing body, compared with running water or the like, the droplets easily and certainly evaporate. Therefore, solar thermal energy can be efficiently transformed to be collected as steam latent heat having a large heat capacity, and the required amount of a medium (the amount of water used) to the amount of heat collected can be greatly reduced. As a result, the light weight and downsizing of the apparatus can be achieved, leading to cost cutting. In addition, since the constructions of the medium supply system and the heat transport system become simple, the system can be easily adapted to a large-scale system for acquiring a large amount of energy, and it can be easily produced and maintained
A solar thermal system, (2) according to the present invention is characterized by comprising a solar thermal collector which comprises an absorbing body, made of a metal plate having one main surface on which a selective absorption film with a large absorptance of solar light and a small emissivity of infrared rays is formed, and the other main surface which is made a black body, a heat insulation case almost in the shape of a box to which the absorbing body is fitted with the other main surface side thereof down, a steam generating chamber formed between the heat insulation case and the absorbing body, a droplet supply means to supply droplets into the steam generating chamber, and a transport system whereby steam generated in the steam generating chamber is transported outside the system, wherein the transport resistance of the transport system is set so that the steam pressure within the steam generating chamber can be kept almost 1 atmospheric pressure.
The difference of the steam pressure within the steam generating chamber from 1 atmospheric pressure changes depending on the thermal insulation performance of the heat insulating material, the cross section and distance of the steam passage, the speed of flow of steam, and the like, but it is desirably plus a few hundred Pa or less, if there is.
Using the above solar thermal system (2), almost the same effects as those in the solar thermal system (1) can be obtained, and solar thermal energy can be efficiently absorbed by the absorbing body with the selective absorption film formed on the one main surface thereof, while the heat transfer of the solar thermal energy absorbed by the absorbing body to the droplets can be certainly conducted because of the treatment of making the other main surface a black body. As a result, the solar thermal energy can be more efficiently collected. In addition, the transport resistance of the transport system is set so that the steam pressure within the steam generating chamber can be kept almost 1 atmospheric pressure. Therefore, as steam is generated within the steam generating chamber, almost the same amount of steam as that can be automatically pushed out and transported in sequence through the transport system by making one side of the transport system open to the atmosphere. Since the sectional form of the transport system can be easily determined based on the steam transport capacity, transport distance, and the like so that the steam pressure within the steam generating chamber can be kept almost 1 atmospheric pressure, the transport system can be easily designed, and the resistance to pressure can be wholly held down, resulting in a large reduction in cost.
A solar thermal system (3) according to the present invention is characterized by comprising: a solar thermal collector which comprises an absorbing body, made of a metal plate having one main surface on which a selective absorption film with a large absorptance of solar light and a small emissivity of infrared rays is formed, and the other main surface which is made a black body, a heat insulation case almost in the shape of a box to which the absorbing body is fitted with the other main surface side thereof down, a steam generating chamber formed between the heat insulation case and the absorbing body, a droplet supply means to supply droplets into the steam generating chamber, and a transport system whereby steam generated in the steam generating chamber is transported outside the system, wherein the transport resistance of the transport system is set so that the steam pressure within the steam generating chamber can be kept almost 1 atmospheric pressure; a steam storage means, being connected through the transport system of the solar thermal collector, with an opening open to the atmosphere in the lower portion thereof and a steam storage portion in the upper portion thereof; a heat-using means such as a heat exchanger and/or a power-using means such as a steam turbine, being connected through a steam piping to the steam storage means; and a discharge means to discharge a gas such as steam introduced to the heat-using means and the steam piping.
Using the above solar thermal system (3), almost the same effects as those in the solar thermal system (2) can be obtained, and the steam storage means with a given-shaped opening is connected through the transport system. Therefore, as steam is continuously generated in the steam generating chamber, almost the same amount of steam as that is automatically pushed out of the transport system in sequence and is transported to the steam storage means. In addition, since the specific gravity of steam is smaller than that of the atmosphere and the atmosphere is pushed up at all times with 1 atmospheric pressure through the opening, the transported steam with 1 atmospheric pressure can be certainly stored within the steam storage means. Since a heat-using means and/or a power-using means are connected through the steam piping to the steam storage means, and a discharge means to discharge a gas such as steam introduced to the heat-using means and the steam piping is equipped, the atmosphere dwelling within the heat-using means and the steam piping at the start of the operation can be discharged, or an incondensable gas generated within the beat-using means during operation can be discharged. As a result, the steam within the steam storage means can be certainly transported and introduced to the heat-using means and/or the power-using means to be utilized.
A solar thermal system (4) according to the present invention is characterized by comprising: a solar thermal collector which comprises an absorbing body, made of a metal plate having one main surface on which a selective absorption film with a large absorptance of solar light and a small emissivity of infrared rays is formed, and the other main surface which is made a black body, a heat insulation case almost in the shape of a box to which the absorbing body is fitted with the other main surface side thereof down, a steam generating chamber formed between the heat insulation case and the absorbing body, a droplet supply means to supply droplets into the steam generating chamber, and a transport system whereby steam generated in the steam generating chamber is transported outside the system, wherein the transport resistance of the transport system is set so that the steam pressure within the steam generating chamber can be kept almost 1 atmospheric pressure; a steam storage means, being connected through the transport system of the solar thermal collector, with an opening open to the atmosphere in the lower portion thereof and a steam storage portion in the upper portion thereof; a heat-using means such as a heat exchanger and/or a power-using means such as a steam turbine, being connected through a steam piping to the steam storage means; a discharge means to discharge a gas such as steam introduced to the heat-using means and the steam piping; and an outside-air intake regulator.
Using the above solar thermal system (4), almost the same effects as those in the solar thermal system (3) can be obtained, and since the outside-air intake regulator is equipped, the amount of steam supplied can be regulated or a prescribed amount of outside air can be introduced, so that the movements of the heat-using means and the power-using means can be controlled and the movement of the heat-using means can be certainly stopped.
A solar thermal system (5) according to the present invention is characterized by the outside-air intake regulator, which is controlled using a controller, based on data from a detecting means to detect the amount of steam generated and the amount of steam stored in the steam storage means in the solar thermal system (4).
Using the above solar thermal system (5), when multiple heat-using means and/or power-using means are connected to the steam storage means, prescribed amounts of flow of steam can be certainly distributed and supplied to prescribed heat-using means and/or power-using means to drive, in accordance with variations in the amounts of steam generated and stored.
A solar thermal system (6) according to the present invention is characterized by a mixing means to mix steam in the steam generating chamber in any of the solar thermal systems (1)-(5).
Using the above solar thermal system (6), droplets and steam which are distributed and dwell so that the temperatures thereof become higher in an upward direction from the bottom within the steam generating chamber can be wholly stirred up and mixed. As a result, the heat transfer efficiency from the absorbing body to the droplets can be heightened, so that steam can be efficiently generated.
A solar thermal system (7) according to the present invention is characterized by the droplet supply means, which also serves as a mixing means in any of the solar thermal systems (1)-(5).
Using the above solar thermal system (7), the mixing means can be arranged at the same place as the droplet supply means, and droplets can be mixed at the same time as they are generated. As a result, the heat transfer efficiency can be further higher.
A solar thermal system (8) according to the present invention is characterized by comprising a transmission body which transmits light arranged above the absorbing body so as to cover the absorbing body in any of the solar thermal systems (1)-(7).
Using the above solar thermal system (8), by setting the distance between the transmission body and the absorbing body to be short, a heat transfer loss outward based on the occurrence of convection can be prevented, and a selective absorption film formed on the top of the absorbing body can be protected mechanically and chemically.